Geographic Variation

Abstract

This chapter focuses on geographic variation in morphology. When morphological variation and range are discontinuous, i.e., the populations or metapopulations are allopatric and can be diagnosed from one or, more commonly, a few characters, they are usually recognized as species, with the inference that they have diverged irrevocably in their evolutionary paths. Mammal species tend to vary geographically most in those features that vary most within a population. If, as for most mammals, body size varies broadly within a population, then geographic variants will usually differ in average body size. In another example, odontocete cetaceans are unusual among mammals in that they vary greatly in the number of teeth and vertebrae within a species, and, as expected, these features differ sharply between geographic forms. It is likely that most geographic variation in morphology in marine mammals is due to differential selection rather than genetic drift. By saying that two populations belong to the same species, we are implying that there is, or recently has been, gene flow between them. Populations can diverge sharply morphologically in the presence of even substantial gene flow if the ecologically engendered differential selection is strong enough.